The typical commercial method for producing hydrogen at present is via steam and/or air reforming of suitable liquid or gaseous hydrocarbon rich fuels. The reforming process involves reacting the hydrocarbon fuel with steam and/or with air or oxygen-enriched air, to produce a syngas stream, which contains hydrogen and also other non-hydrogen by-products including carbon monoxide, carbon dioxide, water, residual hydrocarbon fuel and/or nitrogen. In conventional hydrogen production systems, the carbon monoxide in the syngas stream may be at least partially converted to carbon dioxide by means of the water gas shift reaction to increase the content of hydrogen in the syngas stream, while reducing the content of carbon monoxide (typical high and low temperature water gas shift reactions may reduce the CO concentration in the reformate to about 1% CO).
The development of fuel cell powered vehicles has been pursued in earnest over recent years due to the potential advantages they offer in principle with regards to improved efficiency and emissions reduction. A preferred fuel cell type for this application is the solid polymer electrolyte fuel cell but such cells require a relatively pure source of hydrogen as fuel, with particularly low carbon monoxide levels (typically less than about 50 ppm by volume) to avoid poisoning the anode catalyst. One of the major difficulties to be overcome in fuel cell vehicle development has been in developing a practical means of providing a supply of hydrogen fuel on-board. Hydrogen must either be stored or generated on-board the vehicle but both approaches have faced difficulties. Hydrogen is not easy to store and is usually accomplished either under very high pressures as a compressed gas, as a cryogenic liquid, or adsorbed in heavy, expensive solid alloys. Over the years, attempts have been made to develop compact reformer based subsystems that could process a suitable liquid fuel (e.g. methanol) in order to generate hydrogen on-board. However, satisfactory solutions have proved elusive and, at this time, most fuel cell vehicle developers opt to use gas cylinders in which hydrogen has been compressed to very high pressures (e.g. 5,000-10,000 psi).
Recently however, an improved process for reforming, known as pressure swing reforming (PSR), was invented by Hershkowitz and Deckman. U.S. patent publication No. 2003/0235529 discloses the general construction and operation of pressure swing reformers. The process uses a cyclic, reverse flow reactor which switches between a low pressure combustion step (that heats the reforming catalyst bed) and a high pressure reforming step (that cools the bed). A key improvement associated with this process is that the catalyst bed temperature is hot enough to accomplish the reforming reaction (e.g. >1000° C.) but the reactor inlet and outlet are kept relatively cool (typically <400° C.) thereby simplifying the apparatus. The PSR apparatus therefore may be substantially more compact and less expensive to make than prior art reformers.
A later publication “A Breakthrough Process for the Production of Hydrogen”, B. Kelecom et al, ExxonMobil, 16th World Hydrogen Energy Conference, Jun. 13-16, 2006, discloses how PSR can desirably be used for a hydrogen fuel supply subsystem on-board a fuel cell powered vehicle. Therein, it was demonstrated how PSR apparatus can convert various types of feedstock fuels with very high efficiency and with no apparent deactivation of catalyst. Because of its reduced size and capital cost, a rapid cycle pressure swing adsorption (RCPSA) device is suggested for use in the subsystem in order to separate hydrogen from the PSR product syngas and thereby produce hydrogen of acceptable purity for the fuel cell.
Preferred RCPSA devices typically comprise multiple rotating adsorbent beds in which the beds comprise laminate sheets of immobilized adsorbent. For instance, U.S. Pat. No. 6,565,635 by Keefer discloses suitable compact RCPSAs of such construction. Rotary valves are desirably employed in RCPSAs in order to open and close the adsorbent beds to feed and exhaust the process gases.
The use of two devices employing pressure swing processes in the fuel supply subsystem (PSR and PSA) may offer potential advantages with respect to system integration.